Reticles are masks that are used to control the portions of a material (e.g., an unexposed, undeveloped photoresist) that may be exposed to electromagnetic radiation (e.g., light). In semiconductor device fabrication processes, most of the extremely small features are defined in part by use of reticles, with a large number of different reticles corresponding to different fabrication levels, or elevations, of the semiconductor device.
The reticles that are used in semiconductor device fabrication processes typically include a quartz substrate upon which a radiation blocking layer is fabricated. The light blocking layer includes an opaque, or non-transmissive, material, such as chromium. Attenuated phase shifting reticles may also include one or more layers of partially transmissive, phase shifting materials, such as molybdenum silicide (“MoSi”) or chromium oxide (“CrO”), that phase shift radiation transmitted therethrough (e.g., by 180°). By phase shifting the radiation, diffraction of the radiation may be reduced, improving the resolution of the reticle.
When the reticle is used, radiation is directed through the reticle onto a layer of unexposed, undeveloped photoresist that has been applied to a substrate, such as a semiconductor wafer, which transfers the pattern defined by the light blocking layer and any partially transmissive regions to the layer of unexposed, undeveloped photoresist. The areas of the layer of photoresist that are exposed to radiation passing through the transparent areas and partially transmissive areas of the reticle are typically smaller than the corresponding transparent areas of the reticle. In some cases, the exposed areas of the photoresist have lateral dimensions that are one-fourth the sizes of their corresponding lateral dimensions of the transparent areas of the reticle.
Despite the seemingly increased tolerance that these differences in dimensions may impart to a reticle during its fabrication and use, when a reticle is used in semiconductor device fabrication processes, even very slight damage to the reticle and the presence of very small contaminant particles on transparent and partially transmissive attenuation regions of the reticle may cause imperfections in a photoresist. These imperfections are, of course, transferred to a semiconductor device during its fabrication, and may affect the performance and reliability of the semiconductor device and, ultimately, reduce semiconductor device yields.
One of the prevalent causes of the attraction of contaminants to reticles is the collection of electrostatic energy. The discharge of collected electrostatic energy, or “electrostatic discharge” or “ESD,” is also known to cause damage to the light blocking layers of reticles. ESD is particularly prevalent at the typically tapered corners 27 of attenuation regions 26, as shown in FIG. 1. Although a number of measures have been developed in an attempt to reduce the amount of electrostatic energy to which reticles may be exposed, they are still exposed to some electrostatic energy, which, over time, is collected by reticle features (e.g., metallic light blocking layers).